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Ground Engineering: Roll out the barrels

An unusual legacy of the Second World War has prompted an imaginative remediation strategy in north Kent.

The area around Kingsnorth in Kent is well known for its industrial heritage, so it is no surprise that developer Goodman found contaminants in the ground when it came to construct a new 50ha commercial development near the Medway village. What may be more surprising, however, is the nature of that contamination: 50,000 barrels of tar buried just a few metres down.

“There used to be a whole network of rail lines coming into this area, and it looks like the barrels were brought in by rail and dropped into a big hole.”

“The common story is that they were used for lighting up roadways and runways during the war,” explains Rab McLeod, project manager for contractor Volker Fitzpatrick, which has a £16.5M contract to improve access to the site and build new infrastructure for the development. “There used to be a whole network of rail lines coming into this area, and it looks like the barrels were brought in by rail and dropped into a big hole.”

The undulating grazing land has been settled since the Iron Age, and has a long history of industrial activity, including power generation, as evidenced by large pulverised fuel ash (PFA) lagoons on the east side of the site. However, most of it requires no remediation, and the contractor has been able to carry out a simple earthworks cut/fill exercise to balance up the material on each plot and create level platforms for future development.

Piling not an option

Goodman assumes that future buildings constructed on the PFA will be piled, while plots built on the terrace gravels on the west side will be stabilised with vibro stone columns before construction.

For anyone constructing a building on Plot 2A, however, piling would not be an option, as it is here that the tar barrels have been found. “The barrels are sitting on a 1.5m layer of alluvial clay and then there is an aquifer band over the river terrace gravels,” explains Colin Burford, project manager for demolition and remediation specialist DSM, which has been brought in to clean up the tar contamination. “If you piled through the clay you would take the contaminated water right through to the aquifer.”

Goodman initially considered leaving the barrels in place and putting a capping layer over the top to stop water running off. “We decided that would leave too much of an unknown for anyone buying or leasing that building, so we took the decision to take the material out, treat it and leave it on site,” explains Wellstone Project Management director David Rothwell, who is acting as the client’s representative.

On the spot treatment

The contaminated ground is being treated insitu, using a combination of PFA and cement, with some of the PFA won from the on-site lagoons. Once treated, the material is hauled to the north end of the site and placed in a massive landscaping bund.

Initially the bund was designed to contain 23,000m3 of treated material, but as work progressed it became clear that there were more barrels than anticipated, and the finished bund is likely to contain up to 35,000m3 of the treated material.

As the extent and density of the contamination became clear, DSM developed a strategy of individual processes to treat the ground, which includes the barrels themselves and the soil around them that had become contaminated by tar. In addition to the WWII barrels, the contractor has also found a range of containers with different bituminous products, suggesting the hole originally dug for disposing of the tar barrels was later used to dump by-products of a bitumen processing plant.

“Initially the soil wasn’t soft enough so we had to wet up the pre-dug strip with water to make it into a slurry consistency”

The first stage in the remediation is to remove all the material above the barrels, which consists of crushed concrete, hogging material and PFA. Once this overburden has been stripped to reveal a thin layer of clay/brick capping over the barrels, trial holes are dug every 20m to determine the depth of the barrel layer and to work out how much PFA needs to be added to soak up the tar. This is usually around 700mm thickness of loose PFA or 600mm of insitu PFA to every 1m of barrels.

The next task is to dig down to the underside of the barrels in 5m wide strips. The barrels are removed and placed on the side of the excavation to allow the tar to harden, while the remaining soil and PFA is placed back into the excavation. The tar is then removed from the barrels and also put back.

DSM has brought in a heavy duty shredder, usually seen at waste transfer facilities, to break up the barrels themselves, which are remarkably intact.

Stabilise the mix

The final stage of remediation is to add cement to the mix to stabilise the tar within the PFA/soil matrix, using an Allu PMX 500 excavatormounted mixing arm. “Initially the soil wasn’t soft enough for the mixer to penetrate, because the PFA has too much shear strength, so we had to incorporate a new step in the process and wet up the pre-dug strip with water to make it into a slurry consistency,” says Burford.

Once this is done, cement is added at a rate of between 2% and 2.5% by volume. After mixing, the material is left in place for 24 hours and transported to the north end of the site to be placed in the screening bund, which will eventually be covered with a 600mm layer of topsoil and planting.

DSM expects to finish remediation at the end of this month, and the entire job should be complete by mid-March.

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